Documentation/trace/stm.txt

   1 System Trace Module
   2 ===================
   3
   4 System Trace Module (STM) is a device described in MIPI STP specs as
   5 STP trace stream generator. STP (System Trace Protocol) is a trace
   6 protocol multiplexing data from multiple trace sources, each one of
   7 which is assigned a unique pair of master and channel. While some of
   8 these masters and channels are statically allocated to certain
   9 hardware trace sources, others are available to software. Software
  10 trace sources are usually free to pick for themselves any
  11 master/channel combination from this pool.
  12
  13 On the receiving end of this STP stream (the decoder side), trace
  14 sources can only be identified by master/channel combination, so in
  15 order for the decoder to be able to make sense of the trace that
  16 involves multiple trace sources, it needs to be able to map those
  17 master/channel pairs to the trace sources that it understands.
  18
  19 For instance, it is helpful to know that syslog messages come on
  20 master 7 channel 15, while arbitrary user applications can use masters
  21 48 to 63 and channels 0 to 127.
  22
  23 To solve this mapping problem, stm class provides a policy management
  24 mechanism via configfs, that allows defining rules that map string
  25 identifiers to ranges of masters and channels. If these rules (policy)
  26 are consistent with what decoder expects, it will be able to properly
  27 process the trace data.
  28
  29 This policy is a tree structure containing rules (policy_node) that
  30 have a name (string identifier) and a range of masters and channels
  31 associated with it, located in "stp-policy" subsystem directory in
  32 configfs. The topmost directory's name (the policy) is formatted as
  33 the STM device name to which this policy applies and and arbitrary
  34 string identifier separated by a stop. From the examle above, a rule
  35 may look like this:
  36
  37 $ ls /config/stp-policy/dummy_stm.my-policy/user
  38 channels masters
  39 $ cat /config/stp-policy/dummy_stm.my-policy/user/masters
  40 48 63
  41 $ cat /config/stp-policy/dummy_stm.my-policy/user/channels
  42 0 127
  43
  44 which means that the master allocation pool for this rule consists of
  45 masters 48 through 63 and channel allocation pool has channels 0
  46 through 127 in it. Now, any producer (trace source) identifying itself
  47 with "user" identification string will be allocated a master and
  48 channel from within these ranges.
  49
  50 These rules can be nested, for example, one can define a rule "dummy"
  51 under "user" directory from the example above and this new rule will
  52 be used for trace sources with the id string of "user/dummy".
  53
  54 Trace sources have to open the stm class device's node and write their
  55 trace data into its file descriptor. In order to identify themselves
  56 to the policy, they need to do a STP_POLICY_ID_SET ioctl on this file
  57 descriptor providing their id string. Otherwise, they will be
  58 automatically allocated a master/channel pair upon first write to this
  59 file descriptor according to the "default" rule of the policy, if such
  60 exists.
  61
  62 Some STM devices may allow direct mapping of the channel mmio regions
  63 to userspace for zero-copy writing. One mappable page (in terms of
  64 mmu) will usually contain multiple channels' mmios, so the user will
  65 need to allocate that many channels to themselves (via the
  66 aforementioned ioctl() call) to be able to do this. That is, if your
  67 stm device's channel mmio region is 64 bytes and hardware page size is
  68 4096 bytes, after a successful STP_POLICY_ID_SET ioctl() call with
  69 width==64, you should be able to mmap() one page on this file
  70 descriptor and obtain direct access to an mmio region for 64 channels.
  71
  72 Examples of STM devices are Intel(R) Trace Hub [1] and Coresight STM
  73 [2].
  74
  75 stm_source
  76 ==========
  77
  78 For kernel-based trace sources, there is "stm_source" device
  79 class. Devices of this class can be connected and disconnected to/from
  80 stm devices at runtime via a sysfs attribute called "stm_source_link"
  81 by writing the name of the desired stm device there, for example:
  82
  83 $ echo dummy_stm.0 > /sys/class/stm_source/console/stm_source_link
  84
  85 For examples on how to use stm_source interface in the kernel, refer
  86 to stm_console, stm_heartbeat or stm_ftrace drivers.
  87
  88 Each stm_source device will need to assume a master and a range of
  89 channels, depending on how many channels it requires. These are
  90 allocated for the device according to the policy configuration. If
  91 there's a node in the root of the policy directory that matches the
  92 stm_source device's name (for example, "console"), this node will be
  93 used to allocate master and channel numbers. If there's no such policy
  94 node, the stm core will pick the first contiguous chunk of channels
  95 within the first available master. Note that the node must exist
  96 before the stm_source device is connected to its stm device.
  97
  98 stm_console
  99 ===========
 100
 101 One implementation of this interface also used in the example above is
 102 the "stm_console" driver, which basically provides a one-way console
 103 for kernel messages over an stm device.
 104
 105 To configure the master/channel pair that will be assigned to this
 106 console in the STP stream, create a "console" policy entry (see the
 107 beginning of this text on how to do that). When initialized, it will
 108 consume one channel.
 109
 110 stm_ftrace
 111 ==========
 112
 113 This is another "stm_source" device, once the stm_ftrace has been
 114 linked with an stm device, and if "function" tracer is enabled,
 115 function address and parent function address which Ftrace subsystem
 116 would store into ring buffer will be exported via the stm device at
 117 the same time.
 118
 119 Currently only Ftrace "function" tracer is supported.
 120
 121 [1] https://software.intel.com/sites/default/files/managed/d3/3c/intel-th-developer-manual.pdf
 122 [2] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0444b/index.html